As known, civil transport airplanes should be pressurized, as upon a cruise flight, an airplane flies at an altitude being often higher than 30,000 feet (about 9,000 meters), for which the external air is too low in oxygen (and also too cold and too dry) for being compatible with life. Thus, pressurizing systems are provided in airplanes for keeping on board a breathable atmosphere. In particular, the international aeronautic regulation states that any public transport airplane flying at an altitude higher than 20,000 feet (about 6,000 meters ) should be pressurized and that it should establish in the cockpit an equivalent altitude which does not exceed 8,000 feet (about 2,400 meters) upon a normal flight.
It may however occur that, as a result of a breakdown or a failure, the pressurization of the airplane could no longer be maintained at an acceptable level. A regulatory procedure then compels the pilot to have the airplane descent, as quickly as possible, at a breathable altitude of 10,000 feet (about 3,000 meters) or at the current security altitude if it is not possible to descent as low as 10,000 feet because of the relief. Such a procedure is referred to as an emergency descent.
In such a case, the crew is responsible for different tasks related to initiating the descent, as well as the adjustment of parameters of the descent (speed, target altitude, lateral trajectory, etc.) and this until the airplane flies level at low altitude.
When a crew, as a result of the cockpit becoming decompressed or any other event, carries out an emergency descent, they are requested to deviate from the centre of the air traffic way it followed before the event occurred. Such a measure aims at avoiding that, upon the emergency descent, the aircraft comes into conflict with aircrafts flying along the same air traffic way at lower flight levels. Such an operational requirement is explicitly mentioned in document 7030 of the Civil Aviation International Organization, stipulating that the aircraft having to carry out an emergency descent should deviate from its initial itinerary before starting to descent.
As most of the aircrafts are not provided with automatic systems for carrying out an emergency descent, the whole tasks to be carried out remain the responsibility of the crew, and amongst them, the requirement of deviating from the central axis of the air traffic way upon the initiation of the maneuver. Such a deviation maneuver generally results for the crew in a reflex action via the heading selector of the autopilot. Such an action results in quickly slaving the autopilot on a new heading setpoint, diverging with respect to the initially followed itinerary.
It could happen, however, that in the case of a pressurization loss as a result of which the crew have lost conscience (hypoxia symptoms), the crew is no longer able to apply the above described procedure.
In order to overcome such situations, the emergency descent procedure could be automated.
In particular, from document FR-2,928,465, a particular method is known for automatically controlling an emergency descent of an aircraft. According to this method, when an emergency descent automatic function is triggered, the following successive operations are carried out:
a) a set of vertical setpoints is automatically determined comprising:                a target altitude representing an altitude to be reached by the aircraft at the end of the emergency descent; and        a target speed representing a speed that the aircraft should respect upon the emergency descent;        
b) a set of lateral setpoints is automatically determined, representing a lateral maneuver to be carried out upon the emergency descent; and
c) the aircraft is automatically guided so that it simultaneously respects said set of vertical setpoints and said set of lateral setpoints until reaching said target altitude that it subsequently maintains, said automatic guidance being able to be interrupted by an action of the pilot of the aircraft.
As far as the management of the lateral trajectory within the context of an automated emergency descent is concerned, the following is known:                from document U.S. Pat. No. 4,314,341, an automated emergency descent to a security altitude. In the case of an emergency descent, this document provides automatically applying a rolling setpoint for a predetermined period of time, followed by folding the wings of the airplane flat. Such a maneuver allows to systematically carry out, in the case of an automated emergency descent, a turn with a defined number of degrees (to the left or to the right) and to deviate from the initial itinerary. If this latter maneuver does allow to deviate from the initially followed air traffic way, it could, for instance, guide the airplane in distress toward an area where no deviation ground is available for allowing a landing or toward an area where the relief is more hilly (higher security altitudes), or even toward air spaces wherein aircrafts are not allowed to fly over or even toward air spaces where the traffic is even denser, which is obviously not wanted when the crew is unconscious (hypoxia); and        from document FR-2,906,921, a method for generating a 3D emergency trajectory for an aircraft, being applicable more specifically to situations requiring an emergency descent to be carried out. The device as described in this document allows to create, in addition to a trajectory in the vertical plane, a lateral trajectory leading to the destination of the flight, taking into account relief and performance constraints. Such a solution, however, requires having available, more specifically, perfectly integrated and reliable data bases of the ground (which is not the case currently). It additionally seems particularly tedious and difficult to be industrially contemplated, with respect to the objective to be achieved, that is allowing an airplane to quickly and perfectly safely reach an altitude, at which the occupants of the airplane are able to autonomously breath and without any additional oxygen supply, and allowing a crew that would initially lost conscience to regain conscience so as to ensure the flight to continue until landing on an airport.        
Furthermore, the urgent character of situations leading to implementing an emergency descent does not allow the crew to carry out modifications of the active flight itinerary, via the interface of the flight management system, upon the initiation of the emergency descent. Indeed, such modifications would take some time and require a particular attention from the crew.
Now, managing the lateral trajectory, along which the emergency descent is carried out, shows to be a particularly important element and should more specifically allow:                to minimize the risks of collision with aircrafts likely to fly at lower altitudes on the same air traffic way;        to take into account different flight constraints, being considered by the crew until that point (avoidance of areas of turbulences or dangerous meteorological phenomena);        to stay within the protected sector of the followed air traffic way, such sector for which a security altitude is calculated and published on the navigation maps; and        the air control actors to be able to ensure the safety of the airplane in distress and of surrounding aircrafts.        
Now, as indicated hereinabove, regarding the management of the lateral trajectory upon a non automatic emergency descent, the crew implements simple actions, slaving the autopilot on a selected setpoint, meeting the short term need to deviate from the initial lateral trajectory. Similarly, for aircrafts being already provided with automated systems, managing the lateral trajectory only meets the initial requirement of deviating from the trajectory.
Consequently, none of the usual solutions was able to provide and take into account automatically a lateral trajectory able to meet the different operational constraints of an emergency descent maneuver, and this, whatever the initial situation.
The present invention aims at solving these drawbacks. It relates to an automatic management method of a lateral trajectory of an aircraft upon an emergency descent, said aircraft having to be laterally guided along an initial lateral trajectory.